Avoid creating too many unnecessary build directories for Fantom bundles (#52788)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/52788

Changelog: [internal]

Minor change to just create a single output directory for generated JS code in Fantom per Jest run.

Reviewed By: lenaic

Differential Revision: D78808564

fbshipit-source-id: 70e1a60dfcdcc3fc6ee5f08ced1e9c8f8cab2782